Since man first began the widespread use of refrigeration for preserving foods and similar products, the problem of maintaining the food products in a cool or frozen condition, whether during storage or transport, has been a problem.
Enclosures have been provided which were insulated with various materials in an effort to retain a predetermined interior temperature regardless of ambient temperature. These various insulated enclosures have generally been heavy in weight while at the same time were bulky because of being constructed in final use form.
Knock-down or partially assembled insulated enclosures have heretofore been considered as impractical because they would be too lightweight and heat leakage would occur at the joints thereof which would greatly reduce the efficiency of the unit.